Aluminum and its alloys have long been known to have desirable properties such as low cost, relatively low density and corrosion resistance. Aluminum alloys are widely used in aerospace applications and are typically joined by mechanical fasteners. However, in an increasing number of applications, it is desired to join aluminum parts by welding.
Many space systems are welded, particularly tankage for fuel and oxidizer storage on launch systems. For example, two major systems that require extensive welding are the Titan family of missiles and the External Tank of the Space Shuttle. The Titan family currently uses Aluminum Association (AA) registered alloy 2014, comprising A1, Cu and Mg, as its main structural alloy, which is welded primarily with Al--Si filler alloy AA 4043. The Space Shuttle External Tank, which is essentially huge cryogenic tankage for liquid hydrogen and liquid oxygen, is mostly made of Al--Cu alloy AA 2219 welded with Al--Cu filler alloy AA 2319. In each of these systems, the desirability of saving weight may lead to the use of low density, high strength Al--Li alloys to replace conventional AA alloys 2014 and 2219. Since the conventional alloys, and potentially their Al--Li replacements, are joined by welding, a need exists for a weld filler alloy that can be easily fabricated and welded, and which possesses satisfactory mechanical, physical and corrosion resistance properties in the weldment.
A number of aluminum alloys have been found to have acceptable weldability. Several of these aluminum alloy compositions are based on aluminum-copper systems, wherein significant strengthening is induced by the precipitation of CuAl.sub.2. One such aluminum-copper alloy that is widely used as a weld filler is AA alloy 2319 comprising, in weight percent, Al--(5.8-6.8)Cu--(0.2-0.4)Mn--(0.05-0.15)V--(0.1-0.25)Zr--(0.1-0.2)Ti.
Several recent studies have been performed on the weldability of aluminum-lithium alloys. These include: J. R. Pickens, "The Weldability of Lithium-Containing Aluminum Alloys", Journal of Material Science, Volume 20 (1985); J. R. Pickens, "Recent Developments in the Weldability of Lithium-Containing Aluminum Alloys", Journal of Material Science, Volume 25 (1990); J. C. Lippold, "Weldability of Commercial Aluminum-Lithium Alloys" Proceedings of the Aluminum-Lithium V Conference (1989); and T. S. Srivatsan and T. S. Sudarshan, "Welding of Light Weight Aluminum-Lithium Alloys", Welding Research Supplement (July 1991).
Among aluminum-lithium alloys developed prior to about 1985, the only generally accepted weldable aluminum-lithium alloy is the Soviet alloy 1420 comprising aluminum with 5.0 weight percent Mg and 2.2 weight percent Li. This alloy is reported to have medium to high strength, low density and a modulus of elasticity higher than standard aluminum alloys.
A few aluminum-copper-lithium base alloys, which are typically provided in wrought product form, have been commercialized. These include Aluminum Association registered alloys 2020, 2090, 2091, 2094, 2095, 2096, 2195 and 8090. Although some of these alloys have conventionally been joined by mechanical fasteners for aerospace applications, an increasing need exists for a weld filler alloy that is capable of joining such commercial alloys by welding.
Alloy 2020 has a nominal composition, in weight percent, of Al--4.5Cu--1.1Li--0.5Mn--0.2Cd, and was registered in the 1950's. Although alloy 2020 possessed a relatively low density and developed high strength, it also possessed relatively low levels of fracture toughness and ductility. These problems along with processing difficulties lead to the withdrawal of the alloy from the Aluminum Association register.
Alloy 2090 comprising Al--(2.4-3.0)Cu--(1.9-2.6)Li--(0-0.25)Mg--0.12Zr was designed as a low density replacement for high strength alloys such as 2024 and 7075. Although this alloy has relatively high strength, it also possesses poor short transverse fracture toughness and poor short transverse ductility associated with delamination problems and has not yet experienced widespread commercial useage.
Alloy 2091 comprising Al--(1.8-2.5)Cu--(1.7-2.3)Li--(1.1-1.9)Mg--0.12Zr was designed as a high strength, high ductility alloy. However, at heat treated conditions that produce maximum strength, ductility is relatively low in the short transverse direction. Additionally, the strength achieved by alloy 2091 in non-cold-worked tempers is below the strength attained by the alloy in cold-worked tempers.
Alloy 8090 comprising Al--(2.2-2.7)Li--(1.0-1.6)Cu--(0.6-1.3)Mg--0.12Zr was designed for aircraft applications in which exfoliation corrosion resistance and damage tolerance were required. However, alloy 8090's limited strength capability and less than desired fracture toughness have slowed the alloy's acceptance for widespread aerospace and aircraft applications.
Alloy 2094 comprises Al--(4.4-5.2)Cu--(0.8-1.5)Li--(0.25-0.6)Mg--(0.25-0.6)Ag--0.25 max. Zn--0.1 max. Mn--(0.04-0.18)Zr, while alloy 2195 comprises Al--(3.9-4.6)Cu--(1.0-1.6)Li--(0.25-0.6)Mg--(0.25-0.6)Ag--0.2 max. Zn--0.1 max. Mn--(0.04-0.18)Zr. These alloys possess exceptional mechanical properties such as ultra-high strength, high modulus and high fracture toughness at high strength levels.
U.S. Pat. Nos. 5,032,359, 5,122,339, 5,211,910 and 5,259,897, and U.S. patent application Ser. No. 08/032,158 filed Mar. 12, 1993 and U.S. patent application Ser. No. 08/103,662, filed Aug. 10, 1993, disclose aluminum alloys having excellent properties, which contain copper, lithium, magnesium and other alloying additions.
Each of the above-noted U.S. patents and publications are incorporated by reference herein.
The present invention has been developed in view of the foregoing and provides an Al--Cu--Li alloy with highly improved fabrication and weldability characteristics, as well as favorable mechanical and physical properties.